A photovoltaic cell is a solid state device that converts the energy of sunlight into electricity by the photovoltaic effect. Assemblies of photovoltaic cells are used to make photovoltaic modules or solar panels. Due to the growing demand for renewable energy sources, the manufacture of photovoltaic cells has advanced dramatically in recent years. The most common type of photovoltaic cells is configured as a large-area p-n junction made from bulk materials such as crystalline silicon or polycrystalline silicon.
FIG. 5 shows schematically a cross sectional view of a conventional photovoltaic cell 500. A single p-n junction is formed between a layer of N-type semiconductor 520 and a P-type semiconductor substrate 510. Typically, the layer of N-type semiconductor 520 is formed by ion implantation or surface diffusion of an N-type dopant into the front side of the substrate 510. A depletion or space charge region 550 is formed at the interface where the P-type substrate 510 and the N-type layer 520 meet. In the absence of an external applied voltage, an equilibrium condition is reached when the built-in electric field in the depletion region balances out the diffusion of electrons and holes. A metal electrode 590 is formed on the back side of the substrate 510 and a metal “finger” electrode 570 is formed on the N-type layer 520. The electrodes 590 and 570 are connected to an external load through a wire. As sunlight shines on the single p-n junction from the front side of the substrate 510, electrons in the semiconductor may absorb the light and be excited into the conduction band allowing them to move freely within the semiconductor. The free electrons created in the N-type layer 520 or have been collected by the single p-n junction and swept into the N-type layer 520 may travel through the wire, power the load, and continue through the wire until they reach the P-type substrate where they recombine with holes there. So as to the holes travel through the opposite way to generate the photo-current. Such single junction photovoltaic cells tend to have a relatively low conversion efficiencies, however, usually no more than 18%. This is because the electrons generate at the P-type neutral region or holes generate at the N-type neutral region were quickly recombined, since they were minority carries. However, only the photons absorbed in depletion region or absorbed in neutral region that within electron's or hole's diffusion length, can contributed as photo-current.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.